222 OX THE GEOLOGY AND PETROGRAPHY OF P.ATIIURST, N.S.W., 



Olivine is of course no longer regarded an essential constituent 

 of basalt, but it occurs in such remarkably tine crystals in these 

 rocks that their presence distinguish it at once from all Aus- 

 tralian basalts with which T am acquainted. By taking a micro- 

 photograph and cutting out the portions representing the olivines, 

 the percentage of olivine can be calculated. With the porphyritic 

 crystals this is easily done, but in estimating the granular olivines 

 of the base a large margin for error must be allowed. The applica- 

 tion of this method is common with penologists, and was originally 

 devised by Dr. Sorby.* I have cut several micro-photographs in 

 this way with fairly even results for the average structure of the 



rock. 



Porphyritic olivine ... ... ... 13 percent. 



,, augite ... ... 9 per cent. 



This comparatively large percentage of olivine would bring the 

 rock under Rosenbusch's class of olivine-basalt.f 



Besides the large crystals of olivine there is the granular 

 olivine which with augite and felspar form the base. Under a 

 magnifying power of 100 diameters a micro-photograph can be got 

 of this granular base, from which the parts representing olivine 

 can be cut. My experiences gave me 23 to 29 as the per- 

 centage of this mineral in the base. 



A glance at the micro-photographs appended will show the 

 presence of porphyritic augites. But there is also a very large 

 amount of augites in the micro-granular ground-mass. The 

 quantity of augite is easily shown by treating the slice (after first 

 getting a micro-photograph) with warm hydrochloric acid. After 

 four hours' digestion, the magnetite, serpentinous matters and 

 olivine dissolve, and the felspars and augite only remain ; 

 olivine and magnetite being soluble in HC1., while the augite 

 and plagioclase are scarcely affected. 



* J. J. H. Teall, " Penological Notes on some North of England Dykes." 

 Q.J.G.S.,Vol. xl., p. 21G. 



t H. Rosenbusch, Micro-Physiographie der Massigen Gesteine, Zweite 

 Auflage, p. 733. 



